In order to achieve the demanding goal of Zero Energy, a building must, over the course of a calendar year, generate at least as much energy as it uses.
Shay Brazier and Jo Woods moved into the Zero Energy House early in 2013. In the first year of occupation, the photovoltaic panels in their roof produced twice the energy the couple used at home. The house-wide monitoring system recorded both generation and use. Results for that period showed that the Zero Energy House used 2,361 kWh of electricity and generated 5,387 kWh.
While these results were without a doubt exciting, they were not unexpected. Energy models created during the design phase were used by the couple to size the solar system for their Zero Energy goal, not only in that first year when it was only the two of them living in the house, but also once the family got bigger.
For Shay and Jo, the decision to invest in a photovoltaic (PV) system made economic sense from day one. This was a conclusion they arrived at after doing the math for that first year of living in the Zero Energy House. Their reasons are explained below.
The Zero Energy House is connected to the electricity grid like any other home. At certain times (during periods of high demand or inadequate solar radiation such as nights or cloudy days) the solar system cannot meet all the electricity needs of the house and electricity is bought from a retailer like any other household. However, during most days the PV system generates more than Shay and Jo need, so their grid connection is used to export this surplus and sell it to the same electricity retailer. The grid connection results from 2013 are:
The economic performance of PV in any given year can be determined by comparing the value the system generates to its cost. The economic performance of the Zero Energy House’s solar system is detailed below.
The system value is calculated by adding two figures:
The overall system value, therefore, was $221.51 + $750.34 = $971.85.
This then needs to be compared against the system cost. The system installed in the Zero Energy House was a relatively expensive, roof-integrated solution. Shay and Jo chose this partly for aesthetic reasons, and partly because they wanted to demonstrate what could be achieved with the latest solar solution design.
A standard system with the same rated output (4.16kWp), however, would achieve the same performance and therefore deliver the same value.
Such systems are available for around $12,500. Adding that to a 25-year mortgage along with the rest of the cost to build the house would result in additional mortgage payments of $962 a year.
With the first-year value and cost being basically equal, this shows a PV system this size on a house with this energy profile can pay for itself from the first year of operation. Then, in subsequent years, as energy prices increase the annual cost of the system stays the same but the value it delivers (by avoiding increasingly expensive imported electricity) will increase. This system then starts to return a profit.
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